1. Field of the Invention
The present invention relates to a multiple-image forming apparatus for forming a synthetic image by superimposing image information on a transfer material through the electrophotography or the like.
2. Description of the Related Art
In the case of an image forming apparatus using the electrophotography, it has been performed so far to electrify an electrophotographic photosensitive body serving as an image bearing body by an electrifier, irradiate the photosensitive body with light in accordance with image information to form a latent image, develop the latent image by a development device, transfer an actualized toner image to a sheet material or the like, and form an image.
Moreover, as the number of color images has been increased, a tandem-type image forming apparatus is also proposed which forms a full-color image by preparing a plurality of image bearing bodies in which a series of the above processes is developed, forming a cyanogen image, a magenta image, a yellow image, and preferably a black image on each image bearing body, superimposing the color images at the transfer position of each image bearing body, and transferring them to a sheet material.
The above tandem-type multiple-image forming apparatus is advantageous for speed-up because it has an image forming section for each color.
However, it is a technical problem how to smoothly perform the alignment (registration) of images formed in different image forming sections.
This is because shifts of image forming positions of four colors transferred to a sheet material finally appear as color shifts or a change of color tones.
FIGS. 5A to 5D show types of positional shifts of a transferred image.
As shown in FIGS. 5A-5D, types of positional shifts of a transferred image include a positional shift (top margin) in the scanning-line write direction (direction of the arrow A in FIGS. 5A-5D) against a transfer material 9 (FIG. 5A), a positional shift (left margin) in the scanning direction (direction of the arrow B perpendicular to the direction of the arrow A in FIG. 5B, which is the scanning-line write direction) (FIG. 5B), a positional shift in the diagonal direction (FIG. 5C), and a shift of a magnification error (FIG. 5D), and actually, a shift obtained by superimposing these four types of shifts appears.
Moreover, the main cause of an image shift is a shift of the image write out timing of each image-forming station in the case of the top margin shown in FIG. 5A, and a shift of each image write timing of each image-forming station, that is, a shift of the scanning-start timing in one scanning line in the case of the left margin shown in FIG. 5B. Moreover, the inclination shift in the diagonal direction shown in FIG. 5C is a shift of a scanning-optical-system setting angle or a shift of the angle of the rotation axis of a photo-sensitive drum, and the shift due to a magnification error shown in FIG. 5D is a shift of a scanning-line length due to an error of the optical path length from the scanning optical system of each image-forming station up to the photosensitive drum.
To eliminate the above four types of shifts, the shifts are corrected by adjusting the scan timing of each color for the top margin and left margin. Moreover, the magnification error shift and inclination shift are corrected by the compensator of the conventional multiple-image forming apparatus shown in FIG. 6 by adjusting a pair of mirrors 101 and 102 whose mirror surfaces are perpendicularly held to each other and arranged almost at an angle similar to that of the character `V`, among three turn-back mirrors 101, 102, and 105 disposed in the midst of the optical path of each image-forming station, toward arrows M and N from the apparatus body respectively independently using actuators 103 and 104. In this case, a linear stepping actuator or the like provided with a stepping motor serving as a driving source linearly moving stepwise is used as actuators 103 and 104 for performing the above adjustment.
In the case of the above conventional multiple-image forming apparatus, however, because inclination-shift correction means is complex in configuration and uses many parts, vibrations easily occur. Therefore, the position of a scanning line irradiated onto a photosensitive body is moved to cause scanning irregularity and the scanning irregularity appears as the irregular density of an output image to extremely deteriorate the image quality.
Moreover, these problems greatly influence the formation of a high-quality image free from color shift by a digital color multiple-image forming apparatus.
Therefore, it is an object of the present invention to provide a multiple-image forming apparatus capable of preventing the image quality from deteriorating due to scanning irregularity caused by the inclination shift of a scanning line.